In production of an optical fiber, it is difficult to form a core portion and a cladding portion in a true circular shape, and in a concentric manner, and such portions are formed into a slightly oval shape or a distorted circular shape. Therefore, the refractive index distribution in a cross-sectional structure of an optical fiber is not uniform. As a result, a difference in group velocity of two orthogonally polarized waves in a section of the optical fiber is caused, and there arises a problem in that the PMD (Polarization-Mode Dispersion) is generated. As a method of reducing the PMD, a technique is known in which twists in alternate directions are imparted to an optical fiber.
As a method of imparting a twist to an optical fiber, for example, Patent Reference 1 discloses a method in which a swing guide roller is used. FIG. 11 is a view schematically illustrating the twist imparting method disclosed by Patent Reference 1 above, FIG. 11(A) is a schematic view of the whole method, and FIG. 11(B) is a view illustrating the swing guide roller. As shown in FIG. 11(A), an optical fiber preform 1 which is set in a drawing furnace 2a is heated and softened by a heater 2b, and a glass optical fiber 3 (hereinafter, referred to simply as optical fiber) is drawn. The outer diameter of the drawn optical fiber 3 is measured by an outer diameter measurement unit 4, and fed back to a drawing control unit 5 to control the heating temperature of the heater 2b and the drawing velocity so as to obtain a predetermined outer diameter (usually, 125 μm).
The drawn optical fiber 3 is coated with a coating resin by first and second resin coating dies 6a, 6b, and the resin is cured by first and second ultraviolet irradiation units 7a, 7b. Then, the coated optical fiber 3′ which is coated with the resin is wound around a bobbin 8d through a rod roller 8a, a swing guide roller 9, stationary guide rollers 8b, 8c, and the like.
As shown in FIG. 11(B), when the rotation axis y of the swing guide roller 9 is swung by +θ about the drawing direction axis z, a lateral force is applied to the coated optical fiber 3′ by the swinging operation, and the coated optical fiber 3′ rolls along the surface of the swing guide roller 9. This rolling causes a twist to be imparted to the coated optical fiber 3′. When the swing guide roller 9 is then swung in the opposite direction by −θ, the coated optical fiber 3′ rolls in the opposite direction along the surface of the swing guide roller 9. When the swinging from +θ to −θ is repeatedly applied to the swing guide roller 9 as described above, a clockwise twist and a counterclockwise twist with respect to the traveling direction of the coated optical fiber 3′ are alternately imparted.
Additionally, various methods such as a method in which, as shown in FIG. 12, a twist is imparted to the optical fiber 3′ while the optical fiber is interposed between paired rollers 9a, 9b may be employed.
Patent Reference 2 discloses the swing period and size of a swing guide roller, and also a technique in which the twist imparting to an optical fiber is performed off-line (rewinding and the like) in addition to that in which the spin imparting is performed online during the drawing operation.